1. Field of the Invention
The present invention relates to a coil for an electric rotating machine, which is configured by forming an insulation layer on the periphery of a conductor, and mica tape and a mica sheet used for the coil insulation.
2. Description of the Related Art
In a stator of a high-voltage electric rotating machine such as a generator and a motor, as shown in FIG. 1, a plurality of slots 1a are formed on the inner surface of stator core 1. The each slot 1a accommodates a stator coils 2.
The stator coils 2 comprise an upper coil and a lower coil. To accommodate the upper and lower coils into the slot, insulation spacers 3 are inserted into the bottom of the slot 1a, between the upper and lower coils, and into the opening of the slot 1a. 
Furthermore, a slot wedge 4 for securing the stator coil 2 is inserted in the open end of the slot 1a. The slot wedge 4 suppresses the stator coil 2 vibration generated from electromagnetic pulsating force due to load current in the conductor.
In the above stator of the high-voltage electric rotating machine, the stator coil 2 accommodated in the slot 1a of the stator core 1 is formed of a conductor that has the following structure.
First, a plurality of insulated square strands 21 are bundled and Roebel-transposed. After that, pre-preg separator 11 with thermosetting resin is arranged between a strand bundle 30a on the left side row in the figure and a strand bundle 30b on the right side row in the figure. Pre-preg filler 12 with thermosetting resin is arranged on the Roebel-transposed portion on top and bottom of the strand bundles 30a and 30b. 
Then, the thermosetting resin in the pre-preg separator 11 and that of the pre-preg filler 12 are heated and cured while the strand bundles 30a and 30b are integrally molded in one piece by heat pressing. A conductor 22 is finished to finally have a section as shown in the figure by heating and curing the pre-preg separator 11 and pre-preg filler 12.
The conductor 22 is insulated by the following process called vacuum pressurized impregnation system with thermosetting resin.
A mica tape 7, which includes mica paper 5 and glass cloth backing 6 as shown in FIG. 2, is wound a plurality of layers around the conductor 22. Thermosetting resin is impregnated into the wound layers of the mica tape 7 under vacuum pressurized condition. After that, the thermosetting resin and the mica tape 7 are heated and cured by heat pressing while the stator coil 2 is molded to so as to have a final section. An insulation layer 8 is formed on the periphery of the conductor 22 by heating and curing the thermosetting resin and the mica tape 7.
In the stator coil 2 so configured, the conductor 22 generates heat by load current at the time of the operation of the electric rotating machine. Part of the generated heat is transmitted to the environmental cooling gas directly from ventilation ducts, which are arranged on the cross section of the stator core 1 at appropriate intervals in the axial direction, through the insulation layer 8. Most of the remaining generated heat is transmitted to the stator core 1 through the insulation layer 8 and then indirectly to the environmental cooling gas. When the generated heat is thus transmitted to the cooling gas, all the heat generated from the conductor 22 is cooled via the insulation layer 8 of the stator coil 2. From these points of view, the thermal conductivity along the heat passage is very important.
If the thermal resistance along the heat passage is high, the heat generated in the conductor 22 will be hard to transmit to the cooling gas and thus the temperature of the stator coil 2 will increase to excess. The excessive temperature rise accelerates the deterioration of electrical and mechanical performance of organic materials in the insulation layer 8 as the electric rotating machine is operated for a long time.
The above insulation layer 8 includes mainly mica paper 5, glass cloth backing 6, thermosetting resin, etc. The thermal conductivity of mica is about 0.5 W/m.K, that of glass is about 1.0 W/m.K, and that of epoxy resin, which is a typical thermosetting resin, is about 0.2 W/m.K.
In order to improve the thermal conductivity of the insulation layer 8, therefore, it is effective to reduce the volume of thermosetting resin with the lowest thermal conductivity.
Moreover, the impregnated thermosetting resin can easily be detained in the texture of the glass cloth backing 6 and in the mica paper 5. To prevent the thermosetting resin from being detained, the measures adopting a very thin glass cloth backing 6, squeeze out the impregnating resin by applying an appropriate molding pressure at the time of heating and curing, and the like are taken.
The prior art to positively improve the thermal conductivity of the insulation layer 8 of the stator coil 2 is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 55-53802 (former) and U.S. Pat. No. 4,806,806 (latter).
In the former improved art, inorganic particles such as aluminum oxide and boron nitride having higher thermal conductivity than that of the resin are mixed with mica flakes together with synthetic fibers such as polyamide fibers. The inorganic particles so used each have a diameter of 30 μm to 100 μm. The synthetic fibers serve as a reinforcement member for the mixed mica paper.
In the latter improved art, the inorganic particles of high thermal conductivity are arranged not only in the mica paper but also between layers of the mica tape 7.
Specifically, the following techniques are disclosed:
(1) After a mica tape is wound around a conductor, a resin to which inorganic particles are added is impregnated and then heated and cured in a molding jig.
(2) A so-called pre-preg mica tape that is pre-impregnated with a resin containing inorganic particles is wound around a conductor and then heated and cured in a molding jig.
(3) A so-called pre-preg mica tape that is pre-impregnated with a resin is coated with inorganic particles on its surface and wound around a conductor. After that, it is heated and-cured in a molding jig.
(4) A thin insulation tape coated with inorganic particles is wound around a conductor, together with a mica tape, to form a main insulation layer.
However, there were some problems in taking the conventional measures to improve the thermal conductivity of the stator coil insulated by the above-mentioned vacuum pressurized impregnation method.
In the former method of mixing the inorganic particles with the mica flakes, together with the synthetic fibers such as polyamide fibers, the inorganic particles are buried and dispersed in the mica paper. Thus, the inorganic particles and the synthetic fibers make slits in the mica paper.
Consequently, in the impregnation of resin after the winding of mica tape around the conductor, the slits formed by the inorganic particles facilitate the resin impregnation. On the other hand, the slits formed by the inorganic particles that are present in the insulation layer provided on the peripheral surface of the conductor decrease the electrical strength.
In the case of method (1), or in the latter method of adding high thermal-conductivity inorganic particles to the resin and impregnating resin not only in the mica paper but also between layers of the mica tape, the added inorganic particles will precipitate during the storing of the impregnating resin. Moreover, the inorganic particles could be distributed unevenly as the result of filtering with the mica tape in the impregnation process.
In the case of method (4), a thin insulation tape coated with inorganic particles is wound more layers than necessary. Therefore, the increase of the thickness of the main insulation layer cannot be ignored.
The methods (2) and (3) are so-called pre-preg insulation system using a pre-preg mica tape. If these methods were introduced to the vacuum pressurized impregnation system, the following peculiar problems will arise.
In mica tape for the vacuum pressurized impregnation system, the mica flakes are adhered each another, and mica paper and glass cloth backing are also adhered with a minimum amount of adhesive. The adhesive must be dissolved with the impregnating resin during the impregnation process and they must be formed integrally as a main insulation wall through heat curing. Consequently, the adhesive is required to have mutual dissolubility with the impregnated resin. If, therefore, the impregnated resin is, for example, epoxy resin including a curing agent, the epoxy resin (which may contain an accelerator) is generally selected as an adhesive.
In order to improve the thermal conductivity with the mica tape according to the methods (2) and (3), retaining the inorganic particles arranged on the mica tape with a minimum amount of adhesive and forming a main insulation wall through heat curing after the impregnation process are required.
However, when the mica tape that holds the inorganic particles with the adhesive is wound around the conductor and then impregnated with resin in the impregnation process, the adhesive and the impregnated resin are dissolved each other and the resin viscosity temporarily decreases at the early stage of heat curing. For this reason, a part of resin including the inorganic particles flows out of the insulation layer.
Particularly, in a system that molds the conductor 22 by heat pressing, the squeeze-out of the inorganic particles increases as a molding pressure force to squeeze out an excess impregnated resin from the insulation layer.
The tendency of flow of the inorganic particles is more conspicuous in a system that is impregnated with a low-viscosity, high-solubility resin and molded at a high pressure.
In an insulation system configured by a vacuum pressurized impregnation system as described above, even though a given number of inorganic particles are added in advance to the mica tape, they will flow out during the molding. Therefore, the thermal conductivity cannot be improved as is expected.